How is it so difficult for you to grasp that contemporary news accounts of British ships carrying Southern goods from Southern ports for export overseas, with the actual names of the vessels and manifests of their cargo listed, proof that no one was forced to use Northern ships?
You ignore the loss of income from sailing a ship to a Southern port with an empty cargo hold. That represents loss of money.
Sure, they could do it, but Northern ships didn't have to do that. They could carry cargo on every run and thereby make more money than their competition.
It was economically unfeasible.